1. Field of the Invention
The present invention relates to a method of and an apparatus for preventing scanning line deviation caused by a difference between respective rotational speeds of an original drum and a recording drum in a process scanner of a separated drum type, in which the original drum and the recording drum are driven separately by respective motors.
2. Description of the Prior Art
A so-called drum-type process scanner comprises an original drum on which an original is mounted to be read and a recording drum on which photo-sensitive material is mounted to be exposed. Drum type process scanners are usually classified into a common shaft type having a shaft common to both the original drum and the recording drum, and a separated shaft type (i.e., a separated drum type) having separated shafts for the original drum and the recording drum, respectively. Because a process scanner for scanning and recording a large-sized image must employ input and recording drums of large sizes which are difficult to be construct, and transport, set up, etc., the process scanner for large-sized images is mainly of the separated drum type.
In the process scanner of the separated drum type, the input and recording drums must be driven synchronously with the same rotational speed. Employed therefore are two equivalent synchronous motors rotating in synchronism with the frequency of the AC power driving the motors and two equivalent reduction transmission mechanisms for the respective original and recording drums. This serves to drive the original and recording drums with the same rotational speed.
When each of the reduction transmission mechanisms consists of a gear, a timing belt and the like which have no slip margin, the so-called slow start control under which driving motors are started at relatively low speed and accelerated gradually thereafter must be employed for driving large-sized heavy drums in order to mitigate mechanical shock to the reduction transmission mechanisms. Because the slow start control needs a complicated and expensive control circuit, a transmission mechanism, having a flat belt and a flat belt pulley, which causes little mechanical shock at starting operation, is employed in the process scanner with large-sized drums.
However, it is difficult to produce flat belt pulleys for the original and recording drums to have identical diameters, and therefore, differences between the rotational speeds of the original and recording drums are inevitable. Consequently, a phase difference between the respective rotations gradually changes in proportion to the continuation of the rotations.
In order to compensate for the discrepancies between the rotational speeds and to record a reproduced image which is faithful to the original image, image data obtained by scanning an original is temporarily stored in a memory means for each scanning line, and is then read out to a recording part of the scanner in accordance with rotational phase of the recording drum. Even then, if the phase difference of the rotations gradually changes out of an allowable range, a scanning line deviation finally occurs, and a part of the original image is missed or duplicated on the reproduced image, whereby the reproduced image does not conform to the original image.
Although the change of the phase difference is of small per one rotation cycle, it accumulates over in thousands of rotations during the time required for scanning the whole of the reproduced image. The sum of the differences can cause scanning line deviations.